This application relates in general to systems for assembling components and in particular to a system for sensing and forming leads of components. This application also relates to optical systems for illuminating objects and the sensing of objects.
Industrial robots are increasingly being used for assembly work in manufacturing. In one area of assembly work, leads of electronic components are to be inserted into holes on printed circuit boards. When the leads are separated spatially by the correct distances so that they can fit simultaneously into the holes, the leads are said to be in the "drop-in" condition. Frequently, however, the leads are not in the drop-in condition for a wide variety of reasons. The leads may have been bent during handling, shipping or storage or they may not have been formed properly in the first place. If the leads are not in their proper drop-in condition, it will generally be difficult for a robot to insert the leads of the component into holes on printed circuit boards.
A number of solutions to the above-described problem have been previously proposed. In "Vision System Aligns Leads for Automatic Component Insertion" by Asano et al. in Assembly Automation, February 1983, pp. 32-35 the solution proposed is to cut the leads to different lengths. The longer lead is then aligned with its corresponding hole and inserted therein. A shorter lead is then aligned with its corresponding hole for insertion. If the spacing between the two leads inserted is not the same as the spacing between their corresponding holes, one of the two leads may be bent while the shorter lead is being aligned with its hole. Essentially the same solution has been proposed by Klass in "Circuit Board Assembly Plant to Test Systems" in Aviation Week & Space Technology, Aug. 2, 1982, pp. 66-68, by Sanderson and Perry in "Sensor-Based Robotic Assembly Systems: Research and Applications in Electronic Manufacturing", Proceedings of the IEEE, Vol 71, No. 7, July 1983, pages 856-871 and by Murai et al in "Automatic Insertion of Electronic Components by Optical Detection of Lead Positions", Fourth International Conference on Assembly Automation, pages 390-399.
The above described approach is not entirely satisfactory since it may not be suitable for components with many leads. Furthermore, it takes more effort to prepare these staggered leads. These leads remain staggered after insertion on the other side of the printed circuit board, so that they are non-uniform in appearance and cause quality soldering more difficult.
Another popular approach uses special fixtures into which a robot inserts the leads. All the leads are then bent to one side and then another. After the leads are released, most leads will move back by short distances towards the original positions. Such motion of the leads is known as "spring-back". If the leads have been bent beyond their elastic limit or yield points, they will stop short of moving back to their original positions. Distances for which the leads will move back are determined by many factors referred to below as the spring-back characteristics of the leads. Thus, even though all the leads have been bent to the same positions on one side and then the other, they may move back by different distances so that the lead spacings are still different from those in the drop-in condition.
Yet another conventional approach is to use a specialized gripper which pushes leads inward from an initially outward sprung condition. Where a large number of different types of components must be inserted, a large number of special fingers and grippers may have to be designed for their insertion which may be difficult and uneconomical.
None of the above-described approaches are entirely satisfactory. It is therefore desirable to provide a system for sensing and forming leads so that the leads are in the drop-in condition.
In order to detect the positions of the lead tips, appropriate lighting is necessary so that the cross section of the lead tips reflect light uniformly and a clear visual image can be obtained. In "Automatic Insertion of Electronic Components by Optical Detection of Lead Positions" by Murai et al., Fourth International Conference on Assembly Automation, a circular fluorescent lamp is proposed as the light source for illuminating lead tips. Even when such a source is used, it may be difficult to detect the positions of the lead tips. Thus, as described by Murai et al. in the above article, the body of the electronic component may come out as background in a photograph, particularly when the component body is highly reflective of light. To screen out the noise caused by this background Murai et al. developed an algorithm for obtaining the optimum threshold level for light sensing. Thus, while a circular lamp may be advantageous for some reasons, it has disadvantages since it also illuminates the body of the electronic component which causes background noise. It is therefore desirable to provide other illuminating devices to alleviate such difficulties.